Michael Brown Control Engineering CC 

Practical Process Control Training & Loop Optimisation

CASE HISTORY 185

 

TEMPERATURE CASCADE CONTROL ON BOILER DESUPERHEATERS

 

Desuperheaters are an important element in boilers, and are used to reduce the temperature of superheated steam to a desired value.  The control of them is extremely important.

 

There are often quite complex temperature to temperature cascade control loops used in desuperheating control, and the final output steam temperature is controlled by adding spray water into the superheated steam.  The temperature control of the final temperature is performed using a normal feedback (PID) controller.


On a couple of occasions (one in a large power station in the UK and in another in a petrochemical refinery in South Africa) I have found that the people who implemented the control systems fed the output of the desuperheater temperature controller directly to the desuperheater spray water control valve.  On both occasions I found huge problems occurring with the temperature control after the valve had aged.
 

Essentially the output of the controller (PD) is “demanding” that a certain quantity of water be fed into the desuperheater to satisfy the requirements of the controller as calculated by its PID algorithm, and thus ensure that the temperature process variable (PV) is kept as close as possible to setpoint (SP) with minimum variance at all times. When the PD sends the signal directly to the valve, it is actually “asking” the valve to move to a certain position.  If the valve is sized and set-up correctly and if it has no problems, then the correct volume of water should in fact be fed into the desuperheater.  In reality an average of 75% of valves exhibit problems of many different types, which may get worse as they age.  Therefore if they do have problems, or have not been set up correctly, then they might not feed the correct quantity of water into the steam, as “directed” by the temperature controller.

 
Another important thing to note is that one of the major limitations of feedback control is that you can only tune a controller to control a process as fast as the dynamic response of the process will allow.  This very broadly means that fast processes (like flow) can be tuned with fast control, but slow processes (like most temperatures) can only be tuned with slow control.  If you try and speed up the response of the control system to react any faster to changes such as SP changes or load disturbances (where the PV moves away from SP), then the loop starts cycling.


It is therefore very important that the equipment in the loop used on slow processes works well and without problems, because if the controller has to try and correct for errors caused by equipment like valves, as well as having to respond to changes in error (difference between SP and PV), then the control becomes very difficult and it may take a really extremely long time for the controller to get the PV to the correct value.


To overcome this problem, it is in my opinion really mandatory to use a flow control on the valve with its SP coming from the PD (output) of the temperature controller. (This is known as a cascade secondary flow control).  The temperature control is now actually asking the flow controller to ensure that the process gets the exact volume of water that is required to meet the requirements of the temperature control.  The speed of the flow loop control is very, very much faster than the temperature control, so it can quickly ensure that the valve does in fact deliver the correct amount of water into the desuperheater, and as a result the temperature process is not affected by any of the valve problems.


This works brilliantly and I have helped plants achieve excellent control on slow processes with valves that are not working very well.

 

The first example in this article is taken from a desuperheater temperature controller that was sending its PD signal directly to the valve, and where the temperature control was so bad that the operators were running the loop in manual, with very poor results.  Luckily there was a flowmeter in series with the valve so that we could test the valve performance.

Figure 1.


Figure 1 shows the test done on this valve.  The test shows how the flow through the valve behaved with the signal coming directly from the PD of the temperature controller.   The test showed some remarkable faults:


1. The valve is working very close to seat at around 5% of opening.  This means that the valve is hugely oversized.  Now a well known general rule is that under normal load conditions a valve should never work too close to seat, and as a guideline it suggested that it should be above 20%.  The reasons are that the manufacturers cannot machine the valve plug to give really good linearity when close to seat, and also there might be very huge forces on the plug if the differential pressure across the valve increases as the valve closes which can cause uncontrollable cycling.


2. At the start of the test the controller was in automatic and it can be seen that a continuous cycle was in fact occurring.

 
3. The controller was then switched into manual and a series of step changes made on the PD of the controller.  These showed:


a. In some places the valve actually moved in the opposite direction to the direction of the PD step.
 

b. On many steps there the valve had huge overshoots.
 

c. The valve displayed really seriously bad installed non-linearity.
 

d. The valve displayed extreme stickiness at times.
 

In general it could be concluded that the valve could not provide proper control for the desuperheater temperature, and should be replaced.  It was also recommended that until this should be done they should install a cascade flow control loop, which should be able to keep the flow more or less at the right place to at least get some semblance of temperature control.

Figure 2.

 

To illustrate how this could work, the second example in this article is also of a similar control on another desuperheater where the valve had serious problems, but they were still getting good temperature control on the desuperheater because they had used a cascade flow control with it.


Figure 2 shows a closed loop test on the cascade flow loop, and it can be seen that the loop is in a continuous cycle.  Now although many people always attribute closed loop cycling to bad tuning, there are also many other reasons why it may be cycling.

 

Figure 3.

 

Figure 3 shows the open loop test on the loop.  Two major problems are immediately seen from the test:


I. The valve is approximately four times oversized which can be seen by the relative sizes of steps in PV versus PD.  It must be remembered that oversized valves multiply all the valve problems by the oversize factor.


II. On each valve reversal the PV overshoots quite considerably.  This is known as negative hysteresis which can be a very bad problem as it often causes uncontrollable instability in the loop.  It is caused by either insufficient power in the actuator to overcome the static friction properly, or else by positioner problems.


This is a wonderful example of how the use of a cascade flow control still enabled excellent control of the primary temperature control loop to be achieved in spite of a valve with really severe problems.  If the output of the temperature controller had been fed directly to the valve, there is no way that proper temperature control could have been attained.